Thermal Radiation from an Analytic Hydrodynamic Model with Hadronic and Quark–Gluon Plasma Sources in Heavy-Ion Collisions

Abstract In high-energy heavy-ion collisions, a nearly perfect fluid is formed, known as the strongly coupled quark–gluon plasma (QGP). After a short thermalization period, the evolution of this medium can be described by the equations of relativistic hydrodynamics. As the system expands and cools, the QGP undergoes a transition into hadronic matter, marking the onset of quark confinement. Direct photons offer insights into an essential stage of evolution, spanning from the onset of thermalization to the suppression of thermal photon production, which occurs within the hadronic phase. This paper builds upon and extends a previously published solution of relativistic hydrodynamics, incorporating an equation of state that falls within the same class as that predicted by lattice quantum chromodynamics. Based on this solution, a completely analytic model is constructed to describe thermal photon production, accounting for the quark–hadron transition. The model is tested against PHENIX measurements of nonprompt direct photon spectra in $\text{Au}+\text{Au}$ collisions at $\sqrt{s_{NN}} = 200$ GeV. Good agreement is observed between the model predictions and the experimental data, enabling the investigation of the centrality dependence of the initial temperature. These results provide a benchmark for future theoretical and experimental studies of thermal radiation in heavy-ion collisions.